Raw material powder for molding oxide ion conductor, and method for manufacturing the same

ABSTRACT

It is an object of the present invention to provide a raw material powder for stably obtaining a dense sinter that is prevented from cracking, and a method for manufacturing this powder, and a method for manufacturing a lanthanum-based oxide ion conductor in which this raw material powder is used. 
     The raw material powder manufacturing method of the present invention is a method for manufacturing a raw material powder for forming an oxide ion conductor composed of a multi-component metal oxide including lanthanum or lanthanide, wherein a mixed powder blended such that all of the elements constituting said multi-component metal oxide are included is prefired, after which this prefired powder is exposed to water or moist gas so as to expand at least some of the particles in said powder. Alternatively, two types of mixed powder with different components are prefired separately, after which the prefired powders are blended in a specific ratio.

TECHNICAL FIELD

This invention relates to a method for manufacturing an oxide ionconductor for selectively transmitting oxide ions, and more particularlyrelates to a method for manufacturing an oxide ion conductor composed ofa multi-component metal oxide (composite oxide) containing lanthanoidsuch as lanthanum, and to a raw material powder for forming an oxide ionconductor used in said manufacturing method, and to a method formanufacturing this powder.

BACKGROUND ART

There are known inorganic materials (oxide ion conductors) having theproperty of selectively transmitting oxide ions at high temperature(such as 500° C. or higher). Sinters of multi-component metal oxidesincluding lanthanum or lanthanide (hereinafter also referred to as“lanthanum-based oxide ion conductors”), such as an LaSrCoO₃-basedcomposite oxide or an LaGaO₃-based composite oxide, are known as oxideion conductors with particularly high oxygen transmission performance.These lanthanum-based oxide ion conductors can be utilized inapplications such as separating oxygen from a mixed gas containingoxygen. Some oxide ion conductors are both oxide ion conductive andelectron conductive (the meaning of which includes hole conductivity).Such oxide ion conductors are also called electron-oxide ion mixedconductors (hereinafter also referred to simply as “mixed conductors”).

Types of lanthanum-based oxide ion conductors, and how they are used,are discussed, for example, in the specifications of Japanese Laid-OpenPatent Applications 2001-106532, 2001-93325, 2000-154060, H11-335164,H11-335165, H10-114520, H9-299749, and S 92103, Japanese Patents2,993,639 (Japanese Laid-Open Patent Application H11-253769), 2,966,341(Japanese Laid-Open Patent Application H9-235121), 2,966,340 (JapaneseLaid-Open Patent Application H8-276112), 2,813,596 (Japanese Laid-OpenPatent Application H6-219861), and 2,533,832 (Japanese Laid-Open PatentApplication H6-198149), and U.S. Pat. Nos. 5,306,411 and 5,356,728.

Lanthanum-based oxide ion conductors are generally manufactured by asolid phase reaction method (see the examples (manufacturing examples)in the various publications listed above). This is because the cost ishigher and fewer types of starting raw material can be used with aliquid phase reaction method.

With a solid phase reaction, a mixed powder is prepared by mixing anumber of types of metal compound (oxides or various salts) so that allof the elements constituting the oxide ion conductor will be included.The mixed powder is then prefired within a predetermined temperaturerange. Molding the prefired powder thus obtained into a predeterminedshape and firing it (hereinafter referred to as “main firing”) yields anoxide ion conductor (sinter) of the desired shape.

In order to ultimately obtain a dense sinter (that is, a sinter with astructure dense enough to ensure gas impermeability) in the manufactureof an oxide ion conductor (sinter) by means of a solid phase reaction,it is preferable for the raw material powder that makes up the moldedarticle subjected to the main firing to have higher activity (sinteringreactivity). Accordingly, the temperature at which the mixed powder isprefired is set relatively low (800 to 1000° C., for instance).

However, while the activity (sintering reactivity) of the raw materialpowder will be high if the mixed powder prefiring temperature is low,unreacted particles that have not undergone prefiring (hereinafterreferred to as “impurities”) tend to remain behind in the powder. Theseimpurities can bring about a chemical reaction that leads to asignificant change in volume during the main firing of the moldedarticle. For example, they can bring about a volumetric change in whichfirst swells and then contracts under predetermined conditions.

If such impurities are present in a large quantity in the prefiredpowder, cracks tend to develop in the oxide ion conductor (sinter).Cracking is undesirable in an oxide ion conductor, because gas can passnon-selectively through these cracks and thereby through the oxide ionconductor, the result of which is a decrease in the selective oxide ionpermeation (separation) performance of the oxide ion conductor.

The present invention was conceived in an effort to solve the aboveproblems encountered in the past in relation to a raw material powder(prefired powder) manufactured by solid phase reaction method. It is anobject thereof to provide a raw material powder for stably obtaining adense sinter (oxide ion conductor) that is prevented from cracking, anda method for manufacturing this powder. It is a further object of thepresent invention to use this raw material powder to manufacturelanthanum-based oxide ion conductors of various shapes.

DISCLOSURE OF THE INVENTION

The inventors arrived at the present invention upon discovering thatmarked volumetric change (typically expansion) can occur when impuritiessuch as lanthanum oxide present in the prefired powder react withambient water during firing, and that this volumetric change can be acause of cracking.

One invention provided by the present invention is a method formanufacturing a raw material powder for molding an oxide ion conductorcomposed of a multi-component metal oxide substantially constituted byoxygen and two or more metal elements including at least lanthanum (La)or lanthanide (Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, andLu), comprising the following steps.

The steps comprised by the present invention are preparing a mixedpowder composed of two or more metal compounds blended such that all ofthe elements constituting the multi-component metal oxide will beincluded, prefiring the mixed powder, and exposing the prefired powderto water or moist gas so as to expand at least some of the particles(primary particles and/or secondary particles) in the powder.

With this manufacturing method, after the prefired powder has first beencooled after the prefiring treatment, water is intentionally applied tothe prefired powder to swell the impurities. This brings about areaction that results in a change in the volume of the impurities priorto the main firing (hereinafter referred to as “expansion reaction priorto main firing”). In other words, this prevents a pronounced change inthe volume of some (i.e., the impurities) of the particles (Noparticular distinction is made here between primary and secondaryparticles; and so forth.) that make up the raw material powder duringthe main firing.

Therefore, this manufacturing method makes it possible to provide a rawmaterial powder that is favorable for manufacturing a dense sinter (anoxide ion conductive ceramic) that is prevented from cracking. This step(the step of expanding at least some of the particles in the powder) ispreferably conducted immediately prior to using the raw material powderto mold the oxide ion conductor.

Preferably, the step of expanding at least some of the particles in thepowder is conducted by air drying with normal-temperature or heating at200° C. or lower (typically between room temperature and 200° C.) inmoist air (typically in air with a relative humidity of at least 50%,and preferably a relative humidity of at least 80%) until substantiallyno further weight change is noted in the prefired powder.

With the manufacturing method of this aspect, the expansion reaction ofthe impurities prior to main firing can be brought about by a simpleprocedure. Therefore, a raw material powder that is favorable formanufacturing a dense sinter (an oxide ion conductive ceramic) that isprevented from cracking can be manufactured easily and at low cost.

Also, the step of expanding at least some of the particles in the powderis preferably conducted by stirring the prefired powder under wetconditions, and then performing air drying or heating.

With the manufacturing method of this aspect, moisture can be applieduniformly to the entire mixed powder by wet stirring (hereinafter, “wetstirring” encompasses wet grinding unless otherwise specified). Theexpansion reaction of the impurities prior to main firing can be broughtabout by subsequent air drying or heating. Accordingly, with thismanufacturing method the expansion reaction of the impurities prior tomain firing can be made to proceed substantially simultaneouslythroughout the entire powder. Also, the expansion reaction of theimpurities prior to main firing can be brought about consistentlythroughout the entire powder. Therefore, the manufacturing method inthis aspect makes it possible to manufacture a high-quality raw materialpowder that is favorable for manufacturing a dense sinter (an oxide ionconductive ceramic) that is prevented from cracking.

With the above manufacturing method, it is preferable to use a mixedpowder with the following composition.

Specifically, this mixed powder constitutes a multi-component metaloxide represented by the general formula(Ln_(1-x)A_(x))(B_(1-y)Fe_(y))O₃ (where Ln is at least one selected fromthe group consisting of lanthanum and lanthanides, A is at least oneselected from the group consisting of Sr, Ca, and Ba, B is at least oneselected from the group consisting of Ga, Ti, Ni, Cu, Co, and Mg, 0<x<1,and 0≦y≦1 (typically, 0<y<1)), and is prepared so as to contain an oxideof Ln, an oxide of Fe if needed, an oxide and/or salt of the metal A inthe above general formula, and an oxide and/or salt of the metal B inthe above general formula if needed.

A raw material powder with which a particularly dense sinter (an oxideion conductive ceramic) can be formed can be manufactured by performingone of the above particle expanding steps on this prefired mixed powder.

Another invention provided by the present invention is a method formanufacturing a raw material powder for molding an oxide ion conductorcomposed of a multi-component metal oxide substantially constituted byoxygen and two or more metal elements including at least lanthanum orlanthanide, comprising the following steps.

In order to form an oxide ion conductor whose composition is representedby the general formula (Ln_(1-x)A_(x))(B_(1-y)Fe_(y))O₃ (where Ln is atleast one selected from the group consisting of lanthanum andlanthanides, A is at least one selected from the group consisting of Sr,Ca, and Ba, B is at least one selected from the group consisting of Ga,Ti, Ni, Cu, Co, and Mg, 0<x<1, and 0≦y≦1 (typically, 0<y<1)), a step isperformed of preparing (1) mixed powder containing an oxide of Ln, anoxide of Fe if needed, and an oxide and/or salt of the metal B in theabove general formula if needed, and containing substantially no oxideand/or salt of the metal A, and (2) a mixed powder containing an oxideof Fe if needed and an oxide and/or salt of the metal A, and containingsubstantially no oxide of Ln. The next step is of separately prefiringthese mixed powders of different components. This is followed by a stepof mixing the prefired powders in a predetermined ratio.

With the manufacturing method of this aspect, prefired powders withmutually different components as indicated by (1) and (2) above areprepared separately. In other words, with the manufacturing method ofthis aspect, there is no intermingling of a lanthanoid oxide with anoxide and/or salt of an alkaline earth metal (that is, A) duringprefiring. Accordingly, this prevents the generation of impurities thatcould cause a volumetric change (expansion) during the main firing.Therefore, the manufacturing method of this aspect makes it possible toprovide a raw material powder that is favorable for manufacturing adense sinter (an oxide ion conductive ceramic) that is prevented fromcracking.

With the manufacturing method constituted as above, it is preferable ifthe blending step is performed by mixing the prefired mixed powder of(1) with the prefired mixed powder of (2) under wet conditions, and thisis followed by air drying or heating.

Using a raw material powder obtained by the manufacturing method of thisaspect makes possible the stable manufacture of a highly dense sinter(oxide ion conductive ceramic).

A raw material powder for forming an oxide ion conductor that can befavorably manufactured by one of the manufacturing methods constitutedas above is such that Ln in the above general formula is La, A is Sr,and B is Ga or Ti.

With the manufacturing method of this aspect, it is possible to providea dense sinter (an oxide ion conductive ceramic) that has excellentoxide ion conductivity and high gas impermeability.

As another aspect for solving the stated problems, the present inventionalso provides a raw material powder for forming an oxide ion conductor,obtained by the various manufacturing methods discussed above.

As discussed above, with the raw material powder provided by the presentinvention, expansion reaction of the impurities prior to main firing hasalready been carried out in the course of the manufacture of the rawmaterial powder. Accordingly, this raw material powder can be used toform and manufacture a dense sinter (an oxide ion conductive ceramic)that has excellent oxide ion conductivity and high gas impermeability.

As another aspect for solving the stated problems, the present inventionalso provides a method for manufacturing an oxide ion conductive ceramicby using a raw material powder for forming an oxide ion conductorobtained by the manufacturing method of the present invention asdiscussed above.

Specifically, the method for manufacturing an oxide ion conductiveceramic provided by the present invention is a method for manufacturingan oxide ion conductor composed of a multi-component metal oxidesubstantially constituted by oxygen and two or more metal elementsincluding at least lanthanum or lanthanide, comprising molding into apredetermined shape a raw material powder for molding an oxide ionconductor manufactured by the manufacturing method of the presentinvention as discussed above, and firing this molded article (mainfiring).

With the method of the present invention for manufacturing an oxide ionconductor, it is possible to manufacture a highly-dense lanthanum-basedoxide ion conductor in which substantially no cracks are present.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart showing the results of XRD analysis of the rawmaterial powder of Comparative Example 1; and

FIG. 2 is a chart showing the results of XRD analysis of the rawmaterial powder of Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described.

The present invention relates to technology for the manufacture of a rawmaterial powder (a prefired mixed powder) used to mold a lanthanum-basedoxide ion conductor (oxide ion conductive ceramic) containing at leastlanthanum or lanthanide, and there are no particular restrictions on thetype or composition of the raw material powder as long as it allows themanufacture of a multi-component metal oxide (oxide ion conductor) withsuppressed cracking.

Typically, a raw material powder for obtaining a multi-component metaloxide represented by the general formula a:(Ln_(1-x)A_(x))(B_(1-y)Fe_(y))O₃ can be manufactured with the presentinvention.

The number of oxygen atoms is expressed as 3 in General Formula a, butactually the number of oxygen atoms is 3 or less (and typically lessthan 3). However, the number of oxygen atoms varies with the type ofelements A to C, the value of x and y, and other conditions, and istherefore difficult to express accurately. In view of this, the numberof oxygen atoms is, for the sake of convenience, expressed as 3 in thegeneral formula indicating the multi-component metal oxide in thisSpecification, but this is not intended to limit the technological scopeof the invention taught herein. Therefore, this number of oxygen atomscan also be written as 3-z (so that the above General Formula a isexpressed as (Ln_(1-x)A_(x))(B_(1-y)C_(y))O_(3-z), for example). z hereis typically a positive number not greater than 1 (0<z<1).

In General Formula a above, Ln is at least one selected from the groupconsisting of lanthanum and lanthanides, A is at least one selected fromthe group consisting of Sr, Ca, and Ba, C is at least one metal selectedfrom the group consisting of Ga, Ti, Ni, Cu, Co, Mg, and Fe, B is one ormore metals selected from the group consisting of Ga, Ti, Ni, Cu, Co,Mg, and Fe (but excluding the element selected for C), 0<x<1, and 0≦y≦1(typically 0<y<1).

Ln is preferably La, Ce, Pr, Nd, or Sm, with La being particularlyfavorable. The metal element A in General Formula a is typically justone element (preferably Sr), but can also be two or three elements. Inthis case, the combined stoichiometric ratio for the plurality of A's(such as A1 and A2) becomes x (example:(Ln_(1-x)A1_(x1)A2_(x2))(B_(1-y)C_(y))O₃, x1 +x2 =x).

The metal element B in General Formula a is typically just one element(preferably Ga, Ti, Mg, or Co), but can also be two or more elements. Inthis case, the combined stoichiometric ratio for the plurality of B's(such as B1 and B2) becomes 1-y (example:(Ln_(1-x)A_(x))(B1_(z1)B_(z2)C_(y))O₃, z1+z2=1-y). For instance, it ispreferable if C in General Formula a Fe or Co, and B is Ga or Ti (oneelement), or if C is Fe or Co, and B is a combination of Ga and Mg or Tiand Mg (two elements).

A favorable example of the multi-component metal oxide manufactured withthe present invention is a raw material powder of a multi-componentmetal oxide represented by the General Formula b:(Ln_(1-x)A_(x))(B_(1-y)Fe_(y))O₃ (where Ln is at least one selected fromthe group consisting of lanthanum and lanthanides, A is at least oneselected from the group consisting of Sr, Ca, and Ba, B is at least oneselected from the group consisting of Ga, Ti, Ni, Cu, Co, and Mg, 0<x<1,and 0≦y≦1 (typically 0<y<1)). Ln is preferably La, Ce, Pr, Nd, or Sm,with La being particularly favorable.

The metal element A in General Formula b is typically just one element(preferably Sr), but can also be two or three elements. In this case,the combined stoichiometric ratio for the plurality of A's (such as A1and A2) becomes x (example: (Ln_(1-x)A1_(x1)A2_(x2))(B_(1-y)Fe_(y))O ₃,+x2=x). The metal element B in General Formula b is typically just oneelement (preferably Ga, Ti Co), but can also be two or more elements. Inthis case, the combined stoichiometric ratio for the plurality of B's(such as B1 and B2) becomes 1-y (example:(Ln_(1-x)A_(x))(B1_(z1)B2_(z2)Fe_(y))O₃, z1+z2=1-y).

There are no particular restrictions on the value of x in the aboveGeneral Formulas a and b as long as 0<x<1. x is a value indicating theproportion in which Ln has been substituted with an alkaline earth metal(A) in the lanthanum-based oxide ion conductor (typically a perovskitestructure), and preferably 0.01≦x≦0.95. If the value of x is small (suchas x=0.1 to 0.3), cracks will tend to develop during the main firing,but when the mixed powder (prefired powder) obtained by themanufacturing method of the present invention is used, the result willbe a lanthanum-based oxide ion conductor that is resistant to crackingeven when the value of x is relatively small.

Therefore, the manufacturing method of the present invention isparticularly favorable for manufacturing a raw material powderconstituting a multi-component metal oxide in which 0.01≦x≦0.6 (and evenmore preferably, 0.05≦x≦0.5) in General Formulas a and b above.

There are no particular restrictions on the value of y in GeneralFormulas a and b above as long as 0≦y≦1 (typically 0<y<1) is satisfied.

Next, the powder manufacturing method of the present invention will nowbe described in detail. A person skilled in the art will be able tograsp as prior art design details any matters necessary to the workingof the powder manufacturing method of the present invention other thanthose mentioned in this Specification (such as weighing and blending ofthe various powder raw materials). The present invention can be workedon the basis technological common knowledge in this field and thedetails disclosed by the Specification and Drawings herein.

With the powder manufacturing method of the present invention, variousmetal compounds are mixed in a predetermined ratio by a suitable methodin order to create the multi-component metal oxide to be manufactured.Examples of these metal compounds include oxides of metal elementsconstituting a multi-component metal oxide that becomes an oxide ionconductor (ceramic), and compounds that can become such oxides whenheated (such as carbonates, nitrates, sulfates, phosphates, acetates,oxalates, halides, hydroxides, and oxyhalides of metal elements).

For instance, when the multi-component metal oxide to be manufactured isrepresented by the above General Formula b:(Ln_(1-x)A_(x))(B_(1-y)Fe_(y))O₃ an oxide of Ln, an oxide of Fe, anoxide and/or salt of the metal element A, and an oxide and/or salt ofthe metal element B may be mixed in a predetermined ratio (see theexamples given below).

There are no particular restrictions on how the plurality of types ofmetal compound (preferably in the form of particles) are mixed, but anexample is to prepare a mixed powder by dry pulverization in a planetarymill. Alternatively, the desired mixed powder can also be obtained bywet pulverization in a ball mill or the like while a solvent such aswater or isopropyl alcohol is added. Such powder pulverization andstirring methods are known to persons skilled in the art, and since theyare not particular features of the present invention, they will not bedescribed in detail.

With the powder manufacturing method of the present invention, a singlemixed powder (mixed system) is formed by adding all of the plurality oftypes of metal compound (powder) for creating the desiredmulti-component metal oxide. In this case, a particle expanding step(discussed below) is performed after prefiring.

Alternatively, two or more types of mixed powder (mixed system) in apredetermined combination that produces no impurities may be formed fromthe plurality of types of metal compound particles used to create thedesired multi-component metal oxide.

For instance, the following two mixed systems are produced for themulti-component metal oxide corresponding to General Formula a above.

(1) A mixed powder containing oxide particles of Ln (such as La₂O₃),oxide particles of the metal element C (such as Fe₂O₃), and an oxideand/or salt of the metal element B (such as Ga₂O₃), and containingsubstantially no oxide and/or salt of the metal element A (such asSrCO₃).

(2) A mixed powder containing an oxide particles of the metal element C(such as Fe₂O₃) and an oxide and/or salt of the metal element A (such asSrCO₃), and containing substantially no oxide particles of Ln (such asLa₂O₃) (an oxide and/or salt of the metal element B may be contained, orno oxide and/or salt of the metal element B may be contained).

Separately prefiring two or more mixed systems in such a combinationprevents the occurrence of impurities that could lead to a change involume (expansion) during the main firing.

The mixed powder obtained as above is prefired. Here, a dehydration(solvent removal) step, drying step, crushing step, or the like may beperformed if needed prior to this prefiring step. For instance, when themixed powder is prepared by wet pulverization using a ball mill or thelike, prefiring is generally preceded by dehydration (typically forceddrying at a temperature of 100° C. or higher) and crushing. On the otherhand, when the mixed powder is prepared by dry pulverization, prefiringcan be performed without first performing this drying or crushing.

Just as with a conventional method for manufacturing a powder forforming an oxide ion conductor, the prefiring is typically performed byplacing the mixed powder in a crucible or other such firing vessel.There are no particular restrictions on the prefiring temperature orduration, which may be suitably determined within the range of standardconditions according to the composition of the intended multi-componentmetal oxide. Typically, the prefiring is performed for several hours (1to 3 hours) at 900 to 1300° C.

When two or more mixed systems are to be prepared, they are prefiredseparately as discussed above. The prefiring conditions (temperature,duration, etc.) may be suitably determined according to the compositionsof the mixed systems. The targeted mixed prefired powder is prepared byblending these mixed systems in a predetermined ratio after prefiring.If the prefired powders stick together and form a clump, they may becrushed or pulverized as needed using a mortar, grinder, or the like.

Upon completion of the prefiring step (typically, after cooling to aboutroom temperature), with a single mixed system, a treatment is performedin which the prefired powder is exposed to water or moist gas so that atleast some of the particles in the powder are expanded (hereinafter alsoreferred to as “particle expanding step”).

There are no particular restrictions on the details of this step, aslong as the impurities (such as lanthanum oxide) contained in the powdercan be swollen in the presence of water. For instance, a heat dryingtreatment may be performed in a state in which the prefired powder isexposed to moist gas.

The moist gas here does not need to have any special gas composition,and may be air having an ordinary humidity (typically a relativehumidity of 40 to 95%). An air or inert gas (such as nitrogen) with acomparatively high relative humidity (such as 60 to 100%, and preferablyat least 80%) is preferable. In high-humidity air, either heating isperformed or air drying is performed at room temperature. In the case ofair drying, it is preferably continued for at least 24 hours (andpreferably for 48 to 72 hours). In the case of heating, the preferredcontinuation time will vary with the temperature, but a heatingtreatment at about 50 to 200° C. would be continued for 8 to 12 hours,and preferably 12 to 24 hours, or at least 24 hours. The heatingtreatment is preferably continued until substantially no further changein weight is noted.

This particle expanding step need not be performed immediately after theprefiring, and may instead be performed immediately prior to the moldinginto the predetermined shape (that is, immediately prior to its beingused as a molding material for manufacturing an oxide ion conductor ofthe predetermined shape).

Also, the prefired powder may be exposed to water (the powder may bewetted) in this particle expanding step. Examples of this treatmentinclude wet stirring and wet pulverization using a ball mill, vibratorymill, or the like. For instance, the prefired powder is placed in a ballmill equipped with suitable ceramic balls, and is rotated for apredetermined length of time at a suitable speed. This allows moistureto be applied uniformly to the entire prefired powder. The prefiredpowder can be adjusted to the desired particle size by performing wetpulverization.

If this wet stirring is followed by a heating treatment or air dryingtreatment as discussed above, then it would be performed for long enoughto wet the entire prefired powder. For instance, it can be performed forbetween 0.5 and several hours. Alternatively, if the impurities aresubjected to a swelling reaction during the wet stirring (that is, ifthe above-mentioned heating treatment or air drying treatment isperformed in process of the wet stirring), then the wet stirring wouldappropriately be performed for 6 to 72 hours (preferably 12 to 48 hours,and even more preferably 15 to 40 hours). The wet stirring (typicallywet pulverization) may be performed either at room temperature or underheating (at 50 to 100° C., for example).

The above-mentioned particle expanding step is essential when a singlemixed system of prefired powder is used, but this step may also beperformed when using a plurality of prefired powders (mixed systems)produced by separately prefiring two or more mixed powders as discussedabove. For instance, the wet stirring described above (such as with aball mill) may be employed as the method for blending the two or moreprefired powders (mixed systems), and then air drying or heating may beperformed.

The raw material powder prepared as above, which has undergone expansionreaction prior to main firing, is used to manufacture an oxide ionconductor composed of a multi-component metal oxide.

There are no particular restrictions on the molding of this raw materialpowder and the firing (main firing) of the molded powder, which may beperformed by conventional methods. For instance, uniaxial compressionmolding, hydraulic pressing, extrusion molding, or another standardmethod can be employed. A conventional binder or the like can be addedfor this molding. Components other than the main components of the oxideion conductive multi-component metal oxide (such as the elements in theabove general formula) can be contained to the extent that there is nopronounced decrease in the desired performance (oxygen conductivity,electron conductivity, etc.).

The temperature during the main firing will vary with the composition ofthe oxide ion conductor (multi-component metal oxide) and so forth, butis typically 1200 to 1800° C. (and preferably 1400 to 1700° C.).

The present invention will now be described in further detail throughexamples.

Examples 1 to 3 and Comparative Example 1 below are examples of themanufacture of a raw material powder for forming an oxide ion conductorwith the composition represented by the general formula(La_(0.7)Sr_(0.3))(Ga_(0.6)Fe_(0.4))O₃.

EXAMPLE 1 Single Mixed System, Heating in Air

(a) 23.13 g of La₂O₃, 8.98 g of SrCO₃, 11.41 g of Ga₂O₃, and 6.48 g ofFe₂O₃ were placed in a resin pot along with 400 g of resin balls (100 gof balls 10 mm in diameter, and 300 g of balls 20 mm in diameter), andmixed (dry pulverization) for 5 hours with a planetary mill. Thisprepared a mixed powder.

(b) This mixed powder was transferred to an alumina crucible andprefired for 3 hours at 1000° C.

(c) The prefired powder was placed in a resin pot along with 400 g ofresin balls (100 g of balls 10 mm in diameter, and 300 g of balls 20 mmin diameter), and crushed for 30 minutes with a planetary mill.

(d) Next, a particle expanding step was performed in which this prefiredpowder was exposed to moist gas. Specifically, the crushed prefiredpowder was left for 24 hours in an 80° C. dryer (in an air atmospherewith a relative humidity of 40 to 95%). This manufactured a raw materialpowder.

EXAMPLE 2 Single Mixed System, Wet Pulverization

(a) 101.4 g of La₂O₃, 39.38 g of SrCO₃, 50 g of Ga₂O₃, and 28.4 g ofFe₂O and 28.4 g of Fe₂O₃ were placed in a resin pot (1000 cm³ size)along with 400 g of water and 300 g of YTZ balls with a diameter of 5 mm(trade name for zirconia-based ceramic balls made by NikkatoCorporation), and mixed (wet pulverization) for 15 hours with a ballmill. This prepared a mixed powder.

(b) This mixed powder was dehydrated by ordinary suction filtration.This product was transferred to a metal vat and dried in a 150° C.dryer, and crushed with a mortar.

(c) The crushed mixed powder was transferred to an alumina crucible andprefired for 3 hours at a temperature between 900 and 1250° C.

(d) The prefired powder was crushed with a mortar or grinder.

(e) Next, a particle expanding step was performed in which this prefiredpowder was exposed to water. Specifically, the crushed prefired powderwas placed in a resin pot (100 cm³ size) along with 30 g of water and 30g of YTZ balls, and crushed with a ball mill until the required particlesize was obtained (15 to 40 hours here). This product was dehydrated byordinary suction filtration, after which it was transferred to a metalvat, dried in a 150° C. dryer, and crushed with a mortar. Thismanufactured a raw material powder.

EXAMPLE 3 Preparation of a Plurality of Mixed Systems

(a1) 101.4 g of La₂O₃, 50 g of Ga₂O₃, and 7.1 g of Fe₂O₃ were placed ina resin pot (1 liter size) along with 400 g of water and 300 g of YTZballs with a diameter of 5 mm, and mixed (wet pulverization) for 15hours with a ball mill. This prepared a first mixed powder (hereinafterreferred to as “LGF powder”).

(b1) The LGF powder thus prepared was dehydrated by ordinary suctionfiltration, transferred to a metal vat, dried in a 150° C. dryer, andcrushed with a mortar.

(c1) The crushed LGF powder was placed in an alumina crucible andprefired for 1 hour at 1130° C.

(d1) The prefired LGF powder was crushed using a mortar or a grinder.

(a2) Meanwhile, 39.38 g of SrCO₃ and 21.3 g of Fe₂O₃ were similarlywet-pulverized to prepare a second mixed powder (hereinafter referred toas “SF powder”).

(b2) The SF powder thus prepared was dehydrated by ordinary suctionfiltration, transferred to a metal vat, dried in a 150° C. dryer, andcrushed with a mortar.

(c2) The crushed SF powder was placed in an alumina crucible andprefired for 1 hour at 1130° C.

(d2) The prefired SF powder was crushed using a mortar or a grinder.

(e) Next, the separately prefired and crushed LGF powder (full amount)and SF powder (full amount) were placed in a resin pot along with YTZballs and water, and the LGF powder and SF powder were mixed while beingcrushed with a ball mill until the desired particle size was obtained(15 to 40 hours here). This product was dehydrated by ordinary suctionfiltration, transferred to a metal vat, dried in a 150° C. dryer, andcrushed with a mortar. A raw material powder was manufactured in thisway.

COMPARATIVE EXAMPLE 1 Single Mixed System, no Particle Expanding Step

A raw material powder was manufactured in the same manner as in Example1, except that the particle expanding step was not performed.

Evaluation Test 1: Volumetric Change Caused by Heating the Raw MaterialPowder

Using the raw material powders manufactured in Examples 1 to 3 andComparative Example 1, pellet type molded articles with a diameter ofapproximately 27.5 mm and a thickness of 2 to 2.5 mm were produced bypress molding. These molded articles were heated for 24 hours in an 80°C. dryer (in an air atmosphere with a relative humidity of 40 to 95%).The diameter of the molded articles were measured before and afterheating, and the change in the volume of the molded articles wasevaluated from the change in diameter. The measurement results are givenin Table 1.

TABLE 1 Molded article diameter (mm) Before heating After heatingExample 1 27.53 27.53 Example 2 27.56 27.56 Example 3 27.51 27.51Comparative Example 1 27.52 31.75

As is clear from Table 1, there was substantially no change in thevolume of the raw material powders manufactured by the method of thepresent invention (Examples 1 to 3) before and after heating. On theother hand, the volume of the raw material powder produced inComparative Example 1 changed largely when heated. This revealed thatthe raw material powder of Comparative Example 1 contains a relativelylarge amount of particles that expand markedly when heated.

The main reasons that heating results in less volumetric change in theraw material powders of Examples 1 to 3 will now be described. InExamples 1 and 2, the prefired powder was subjected to a particleexpanding step. As a result, it is believed that any impuritiescontained in the prefired powder and that could undergo a volumetricchange when heated (La₂O₃ in this case) had almost already undergone aexpansion reaction prior to the main firing during the manufacture ofthe raw material powder. In Example 3, an LGF powder and an SF powderwere prefired separately, after which the two powders were mixed. Thisis believed to have prevented the production of any impurities thatcould undergo a volumetric change when heated.

The raw material powders manufactured in Example 3 and ComparativeExample 1 were also subjected to XRD analysis. The analysis results forComparative Example 1 are given in FIG. 1, and those for Example 3 inFIG. 2. FIG. 1 shows peaks indicating La₂O₃ (labeled with a □ mark inFIG. 1). This tells us that the raw material powder manufactured inComparative Example 1 contained La₂O₃ as an impurity. Meanwhile, FIG. 2shows no peaks indicating La₂O₃. This tells us that the raw materialpowder manufactured in Example 3 contained substantially no La₂O₃.

Evaluation Test 2: Evaluation of Oxide Ion Conductor Obtained by Firingthe Raw Material Powder

Using the raw material powders manufactured in Examples 1 to 3 andComparative Example 1, pellet type molded articles were produced in thesame manner as in Evaluation Test 1. These molded articles were firedfor 6 hours at 1500° C. to manufacture an oxide ion conductor molded inthe form of pellets.

The oxide ion conductors thus obtained were visually checked for cracks.Also, the fired pellets were cut into a quadrangular shape (10 mm×20mm), and the conductivity σ (S/cm) of the oxide ion conductor wasmeasured by the following method.

Conductivity Measurement Method

The surface of the oxide ion conductor was coated with a platinum paste(serving as an electrode), after which a platinum wire was connected andbaked on at 850 to 1100° C. The conductivity σ (S/cm) was found bymeasuring the resistance of this oxide ion conductor at 800° C.

Table 2 shows whether any cracks developed and the results of measuringconductivity for the oxide ion conductor manufactured from each of theraw material powders.

TABLE 2 Cracks Conductivity σ (S/cm) Example 1 no 3.14 Example 2 no 5.52Example 3 no 5.10 Comparative yes could not be measured Example 1

As shown in Table 2, no cracks were seen in any of the oxide ionconductors obtained from the raw material powders manufactured inExamples 1 to 3. These oxide ion conductors also exhibited goodconductivity. On the other hand, there were cracks in the oxide ionconductor obtained from the raw material powder manufactured inComparative Example 1, and the conductivity could not be measured. Thisindicates that the raw material powders obtained by the manufacturingmethods of Examples 1 to 3 are favorable as raw material powders forforming an oxide ion conductor (and particularly the above-mentionedmixed conductors).

In Reference Examples 1 to 3 below, a raw material powder for forming anoxide ion conductor composed of a multi-component metal oxidesubstantially constituted by oxygen and two or more metal elementsincluding lanthanum were manufactured by conventional methods, these rawmaterial powders were molded, and the change in their volume afterheating was evaluated.

REFERENCE EXAMPLE 1

Using various metal compounds of La, A (Sr, Ca, or Ba), Ti, and Fe inratios such that the molar ratios of the various metal elements were asshown in Tables 3 to 5, raw material powders (Samples 1 to 18) forobtaining multi-component metal oxides of various compositionsrepresented by the general formula (La_(1-x)A_(x))(Ti_(1-y)Fe_(y))O₃(where A is Sr, Ca, or Ba, x is 0.1 to 0.9, and y is 0.1 to 0.9) weremanufactured in the same manner as in Comparative Example 1 (that is,without performing a particle expanding step).

For each of these raw material powders, the diameter of the moldedarticles before and after heating (for 24 hours at 80° C. in an airatmosphere) was measured in the same manner as in Evaluation Test 1. Theresult was used to evaluate the change in volume of the molded articles.The measurement results are given in Tables 3 to 5.

TABLE 3 Metal element composition Diameter of Sample (molar ratio)molded articles (mm) No. La Sr Ti Fe Before heating After heating 1 0.10.9 0.9 0.1 27.5 27.5 2 0.5 0.5 0.1 0.9 27.53 27.56 3 0.5 0.5 0.9 0.127.51 27.53 4 0.7 0.3 0.1 0.9 27.53 31.25 5 0.9 0.1 0.1 0.9 27.5 32.68 60.9 0.1 0.9 0.1 27.52 32.81

TABLE 4 Metal element composition Diameter of Sample (molar ratio)molded articles (mm) No. La Ba Ti Fe Before heating After heating 7 0.10.9 0.1 0.9 27.52 27.56 8 0.1 0.9 0.9 0.1 27.51 27.54 9 0.5 0.5 0.1 0.927.5 27.53 10 0.5 0.5 0.9 0.1 27.53 27.54 11 0.9 0.1 0.1 0.9 27.51 33.1412 0.9 0.1 0.9 0.1 27.54 33.46

TABLE 5 Metal element composition Diameter of Sample (molar ratio)molded articles (mm) No. La Ca Ti Fe Before heating After heating 13 0.10.9 0.1 0.9 27.54 27.54 14 0.1 0.9 0.9 0.1 27.52 27.53 15 0.5 0.5 0.10.9 27.52 27.57 16 0.5 0.5 0.9 0.1 27.55 27.58 17 0.9 0.1 0.1 0.9 27.5433.16 18 0.9 0.1 0.9 0.1 27.52 32.94

As is clear from Tables 3 to 5, with a composition in which the value ofx in the general formula (La_(1-x)A_(x))(Ti_(1-y)Fe_(y))O₃ is relativelysmall (that is, the molar ratio of La is relatively high), thevolumetric change (expansion) of the molded article after heating isparticularly large. Therefore, using a raw material powder obtained bythe powder manufacturing method of the present invention becomes verysignificant in the manufacture of an oxide ion conductor composed of amulti-component metal oxide with such a composition (particularly whenx=0.1 to 0.3). A tendency for heating to cause volumetric expansion isseen even with compositions in which the value of x is relatively large,and applying the manufacturing method of the present invention to suchcompositions (such as those in which 0.3<x≦0.9) suppresses cracking.

REFERENCE EXAMPLE 2

Using various metal compounds of La, Sr, Ti, Mg, and Co in ratios suchthat the molar ratios of the various metal elements were as shown inTable 6, raw material powders (Samples 19 to 22) for obtainingmulti-component metal oxides of various compositions represented by thegeneral formula (La_(1-x)Sr_(x))(Ti_(z1)Mg_(z2)Co_(y))O₃ (wherez1+z2=1-y, x==0.2 to 0.7, and y=0.35 to 0.6) were manufactured in thesame manner as in Comparative Example 1 (that is, without performing aparticle expanding step). The composition of these multi-component metaloxides corresponds to when Ln=La, A=Sr, B1=Ti, B2=Mg, and C=Co in thegeneral formula (Ln_(1-x)A_(x))(B1_(z1)B2_(z2)C_(y))O₃.

For each of these raw material powders, the diameter of the moldedarticles before and after heating (for 24 hours at 80° C. in an airatmosphere) was measured in the same manner as in Evaluation Test 1. Theresult was used to evaluate the change in volume of the molded articles.The measurement results are given in Table 6.

TABLE 6 Metal element composition Diameter of Sample (molar ratio)molded articles (mm) No. La Sr Ti Mg Co Before heating After heating 190.3 0.7 0.65 0.265 0.085 27.53 27.54 20 0.4 0.6 0.6 0.315 0.085 27.4927.51 21 0.6 0.4 0.5 0.415 0.085 27.52 27.53 22 0.8 0.2 0.4 0.515 0.08527.63 33.82

As is clear from Table 6, heating caused volumetric expansion in all ofthe Samples 19 to 22. Pronounced volumetric expansion was seen withSample 22 in which the value of x was relatively small (x=0.2). Thepresent invention can be favorably applied to the manufacture of anoxide ion conductor composed of a multi-component metal oxide with acomposition such as this (particularly when x is 0.2 or less).

REFERENCE EXAMPLE 3

Using various metal compounds of La, Sr, Ga, Ti, Mg, Co, and Fe inratios such that the molar ratios of the various metal elements were asshown in Table 7, raw material powders (Samples 23 and 24) for obtainingmulti-component metal oxides of various compositions represented by thegeneral formula (La_(1-x)Sr_(x))(B1_(z1)Mg_(z2)C_(y))O₃ (where B1 is Gaor Ti, is Co or Fe, z1+z2=1-y, x=0.2, and y=0.115 to 0.15) weremanufactured in the same manner as in Comparative Example 1 (that is,without performing a particle expanding step). The composition of thesemulti-component metal oxides corresponds to when Ln=La, A=Sr, B1=Ga orTi, B2=Mg, and C=Fe or Co in the general formula(Ln_(1-x)A_(x))(B1_(z1)B2_(z2)C_(y))O₃.

For each of these raw material powders, the diameter of the moldedarticles before and after heating (for 24 hours at 80° C. in an airatmosphere) was measured in the same manner as in Evaluation Test 1. Theresult was used to evaluate the change in volume of the molded articles.The measurement results are given in Table 7.

TABLE 7 Diameter Metal element of molded composition articles (mm)Sample (molar ratio) Before After No. La Sr Ga Ti Mg Co Fe heatingheating 23 0.8 0.2 0.8 — 0.115 0.085 — 27.51 32.92 24 0.8 0.2 — 0.80.15  — 0.05 27.5  34.87

As is clear from Table 7, heating caused pronounced volumetric expansionin both of the Samples 23 and 24. The present invention can be favorablyapplied to the manufacture of an oxide ion conductor composed of amulti-component metal oxide with a composition such as this(particularly when x is 0.2 or less).

Specific examples of the present invention were described in detailabove, but these are merely examples, and do not serve to limit thescope of the claims. The technology discussed in the claims encompassesvarious modifications and changes to the specific examples given above.

Also, the technological elements described in this Specification andDrawings exhibit technical usefulness either alone or in variouscombinations, and are not limited to the combinations given in theclaims at the time of application. Also, the technology illustrated inthis Specification and Drawings simultaneously achieves a plurality ofobjects, and achieving any one of these has technical usefulness byitself.

1. A method for manufacturing a raw material powder for molding an oxideion conductor composed of a multi-component metal oxide represented bythe following general formula:(Ln_(1-x)A_(x))(B_(1-y)Fe_(y)O) ₃ where Ln is at least one selected fromthe group consisting of lanthanum and lanthanides, A is at least oneselected from the group consisting of Sr, Ca, and Ba, B is at least oneselected from the group consisting of Ga, Ti, Ni, Cu, Co, and Mg, 0<x<1,and 0≦y≦1, the method comprising: preparing a mixed powder composed ofmetal compounds so as to contain an oxide of Ln; an oxide of Fe ifneeded; an oxide and/or salt of the metal A; and an oxide and/or salt ofthe metal B if needed; prefiring said powder; and expanding at leastsome particles in said prefired powder by exposing said prefired powderto moist gas and by heating at about 50 to 200° C. until there issubstantially no further change in the weight of said prefired powder inmoist gas.
 2. The manufacturing method according to claim 1, wherein,the value of y in the general formula is more than 0 and less than
 1. 3.The manufacturing method according to claim 1, wherein, in the generalformula, Ln is La, A is Sr, and B is Ga or Ti.